LED lighting systems are capable of high light output while consuming significantly less power than that consumed by traditional incandescent bulbs. LEDs require a tightly regulated current supply, however, and thus LED lighting systems require more complex driver electronics than traditional systems in order to supply this current. In addition, LED lighting systems may be designed to interface with existing lighting infrastructure, such as traditional dimmer switches. A typical LED lighting system thus includes one or more LEDs and driver circuitry to power and dim the LEDs.
One such traditional dimmer switch operates via phase dimming, in which a varying portion of a current or voltage (such as an AC mains signal) is removed or “chopped,” thereby delivering a variable amount of power to the downstream LEDs. A TRIAC may be used to perform this chopping; the amount of the AC mains that is passed is referred to as the conduction angle, or phase, which may be measured in degrees; the leading edge or the trailing edge of the AC mains may be chopped.
FIG. 1 illustrates a sample circuit 100 for a phase dimmer. A voltage V1 supplies power to the circuit 100, and a resistor R1 represents the variable load of a lamp. A second resistor R2 sets the conduction angle; the resistor R2 may be a variable resistor and may have a resistance ranging from 100 kΩ to 10 MΩ or any other value. When the voltage V1 is below a threshold, the TRIAC U1 does not conduct, and a voltage thereby develops across the resistor R2 and a capacitor C1 (the rate of development being controlled by the sizes of R2 and C1). This developing voltage charges the capacitor C1 until the voltage increases sufficiently to cross the conduction threshold of the TRIAC U1 and a second TRIAC U2. The first TRIAC U1 turns on, allowing larger currents to conduct through the lamp resistance R1. The TRIAC U1 continues to conduct until current no longer flows through it, at which point it resets.
The size of the lamp resistance R1 affects the operation of the phase dimmer circuit 100. A traditional incandescent bulb has a resistance of approximately 240Ω (given a 60 W bulb operating at 120 V) in its steady-state condition, though when the lamp is first turned on its resistance may be much higher. The size of the variable resistance R2 (and of the other components in the circuit 100) can thus be set accordingly to ensure that the circuit 100 operates properly. LEDs, on the other hand, may vary in power consumption from anywhere from 4 W to 20 W, which corresponds to an equivalent resistance ranging from 700Ω to 3600Ω. Furthermore, the drivers for LEDs may not have a purely resistive input characteristic, creating further variation in the equivalent resistance. The deleterious effect of this variation can produce a momentary loss of current in the TRIAC U1, resulting in a mis-firing thereof. The effects of this mis-firing include flickering, shimmering, or other dimming incompatibilities of the LED.
Existing systems reduce or prevent these deleterious effects by adding a so-called “bleeder” circuit to draw current through the TRIAC U1 to prevent its misfiring. For example, a resistor may be switched into the circuit 100 or a semiconductor may be operated in its linear region across the input terminals of the driver or LED. The bleeder circuit may be configured to continually draw current or may be configured to draw current only when the current through the TRIAC U1 falls below a threshold and/or only during certain portions of the input AC mains cycle. In either case, however, the bleeder circuit wastes power by drawing this current. A need therefore exists for an LED driver that prevents TRIAC misfire without consuming unnecessary power.